Auditory System Physics Of Hearing

The ability to hear sounds depends on the movement of different parts of the hearing mechanism, beginning with the tympanic membrane, or eardrum. This movement triggers movements in the other structures of the middle and inner ear, and in the cochlea. In turn, these movements trigger nerve impulses to the brain.

The ear differentiates between sounds based on frequency (pitch), and intensity (loudness). A basic understanding of the physics of hearing and sound helps to make the function of the ear’s structures and the processing of sound easier to understand.

Sound Intensity

Sound intensity is perceived as the “loudness” of a sound. Intensity is measured in decibels (dB). The decibel scale begins at 0 dB. This does not mean absence of sound, but rather, it is the point that represents threshold of human hearing (the lowest point at which sound can be perceived by the average person). Sounds of greater or lesser intensity are expressed (using the decibel scale) in comparison to this reference point.

The decibel scale is a logarithmic scale, meaning the intensity of sound multiplies exponentially as decibel measurement increases. Humans can hear a wide range of sound intensities, so the decibel scale is based on multiples of ten, to make measured numbers more manageable. For example, 10 dB is ten times the intensity of 0 dB; however, 20 dB is 100 times the intensity of 0 db. Thus, the decibel scale increases by exponential base ten multiples. In this way, 80 dB is not 80 times as intense as 0 dB, but rather 100,000,000 times the intensity. Sound that is too intense, usually greater than 85 dB, can cause damage to the hearing mechanism. This process is known as noise-induced hearing loss.

Sound Frequency

Frequency is the number of cycles per second in a sound wave, or the number of times the sound wave completes a complete cycle, or period, in one second. Frequency is measured in hertz (Hz), and is perceived by the human ear as pitch. A high number of cycles per second, or high-frequency sound — e.g. 8000 Hz — is perceived as “high pitched.” A low number of cycles per second — e.g. 500 Hz — is perceived as “low pitched.”

Frequency is also related to sound wavelength. These two qualities have an inverse relationship; that is, as frequency (cycles per second) increases, wavelength decreases. In order to repeat a greater number of times in one second, a sound’s wavelength must be relatively short. The higher the number of cycles per second, the higher the perceived pitch and the shorter the wavelength.

Measuring Sound Frequency - Hearing and Sound

In hearing loss, one sound frequency can be affected more than another. For example, you may have a high-frequency hearing loss, in which sounds at higher frequencies (e.g. 4000-8000 Hz) are more difficult to hear, and may require amplification. In other cases, however, all frequencies may be similarly affected.

Speech sounds contain a complex combination of frequencies and intensities; therefore, hearing loss can have a complex effect on speech perception and communication.

Resources

Martin, F.M.